Elevated blood cholesterol is a major health problem in the United States. Hypercholesterolemia is treated aggressively with drugs such as Lipitor™ and dietary restrictions. Lipitor is a trademark of Pfizer Inc. The concept of reverse cholesterol transport proposes that excess cholesterol present in peripheral tissues is returned to the liver for excretion by a process utilizing high density lipoproteins (HDL) or specific subclasses of HDL. It is generally accepted that the protective effect of HDL on the development of atherosclerosis can be attributed to its participation in the reverse cholesterol transport process. The first step in reverse cholesterol transport is the movement of cholesterol from the plasma membrane of cells onto an extracellular HDL acceptor particle. It is generally believed that an increase in the amount of the HDL or an increase in the HDL cholesterol-acceptor efficiency is beneficial in protecting individuals against the development of atherosclerotic lesions.
Recently, a variety of experiments conducted in numerous laboratories have identified two receptors that participate in the transport of cellular cholesterol onto HDL acceptor particles. These receptors are present on the plasma membrane of some cells. One of these receptors, scavenger receptor class B type I (SR-BI) has been shown to bind HDL and to mediate the movement of cholesterol between HDL and the cellular plasma membrane. There is growing evidence indicating that larger, mature HDL particles preferentially interact with SR-BI and facilitate the movement of cholesterol between the HDL and the cell. Other investigations have identified a second receptor that participates in the synthesis of HDL. This receptor, ATP binding cassette protein 1 (ABCA1), has been demonstrated to play a critical role in the synthesis of nascent HDL particles.
Assays have been described to measure ABCA1 and SR-BI mediated cholesterol efflux. Publications that describe various configurations and applications of these assays include: Yancey, P. G., M. de La Llera-Moya, S. Swarnakar, P. Monzo, S. M. Klein, M. A. Connelly, W. J. Johnson, D. L. Williams, and G. H. Rothblat, “HDL Phospholipid Composition is a Major Determinant of the Bi-directional Flux and Net Movement of Cellular Free Cholesterol Mediated by Scavenger Receptor-BI (SR-BI),” J. Biol. Chem. (2000); Bortnick, A. E., G. H. Rothblat, G. Stoudt, K. L. Hoppe, L. J. Royer, J. McNeish, and O. L. Francone, “The Correlation of ABCA1 mRNA Levels with Cholesterol Efflux from Various Cell Lines,” J. Biol. Chem. 275:28634–28640 (2000); Rothblat, G. H., M. de La Llera-Moya, V. Atger, G. Kellner-Weibel, D. L. Williams, and M. C. Philips, “Cell Cholesterol Efflux: Integration of Old and New Observations Provides New Insights,” J. Lipid Res. 40:781–796 (1999); de La Llera-Moya, M., G. H. Rothblat, M. A. Connelly, G. Kellner-Weibel, S. W. Sakr, M. C. Phillips, and D. L. Williams, “Scavenger Receptor BI (SR-BI) Mediates Free Cholesterol Flux Independently of HDL Tethering to the Cell Surface,” J. Lipid Res. 40:575–580 (1999); de La Llera-Moya, M. V. Atger, J. L. Paul, N. Fournier, N. Moatti, P. Giral, K. E. Friday, and G. H. Rothblat, “A Cell Culture System for Screening Human Serum for Ability to Promote Cellular Cholesterol Efflux: Relationships Between Serum Components and Efflux, Esterification and Transfer,” Arterioscler. Thromb. 14:1056–1065 (1994). Although the assays to measure ABCA1 and SR-BI-mediated cholesterol efflux have been published, they have been used individually and generally applied to study isolated HDL or other acceptor particles. It is desirable to provide an improved method for determining cholesterol efflux potential for serum.